To have Jest produce a coverage report, showing what parts of your code were (and weren't) covered by your tests, all you have to do is add a pair of parameters to the corresponding script in the package.json file:

 "test": "jest out/ --coverage --no-cache"

In the preceding line of code, the first parameter, --coverage, tells Jest to collect all of the necessary information, and the second parameter, --no-cache, ensures that all information will be fresh; in certain situations, not totally correct results have been produced when this parameter was omitted. How does this affect the testing? Let's see!